1. Field of the Invention
The present invention relates to the preparation of vinyl ester resins with low volatile organic compound (VOC) content to reduce hazardous air pollutant (HAP) emissions. More specifically, the present invention relates to multi-modal vinyl ester resins.
2. Brief Description of the Prior Art
Styrene is the most common reactive diluent used in thermosetting liquid molding resins. Recently, the Federal Environmental Protection Agency of the United States of America introduced legislation to address hazardous emissions from composite manufacturing and repair by enacting new emission standards through the “National Emission Standards for Hazardous Air Pollutants: Reinforced Plastic Composites Production,” which specifically targets styrene, methyl methacrylate, and methylene chloride as regulated hazardous air pollutants. Volatile organic compound emissions are liberated during all of the phases of composite fabrication. Styrene emissions occur during the mixing of diluents, catalysts, and initiators into the system. Composite parts typically have very large surface to volume ratios, which allows up to 20% of the styrene content to be lost during the molding stage. During cure, elevated temperatures increase the vapor pressure of styrene and thus increase the rate of VOC emissions. Unfortunately, even after cure during the lifetime of the part, styrene emissions can be substantial. Past work has shown that up to 50% of the styrene is unreacted after cure. Therefore, liberation of VOC emissions must be mitigated not only during composite processing, but also during curing and fielding of the composite part.
Simply reducing the styrene content in VE resins causes two problems. First of all, the resin viscosity increases unacceptably. The second problem is that the fracture toughness of these resins decreases as the styrene content is reduced. Lower molecular weight vinyl ester monomers can be used to reduce the resin viscosity, but detrimentally affects the fracture properties. This is the problem with Dow Derakane™ 441-400, which uses low molecular weight vinyl ester monomers and only 33 wt % styrene. The viscosity is approximately 400 cP, which is acceptable for liquid molding operations. On the other hand, the fracture toughness is only ˜100 J/m2.
Reactive diluents other than styrene have been used to reduce both VOC and HAP emissions. 2-hydroxymethacrylate has been used, but the resin viscosity and properties of the resulting polymers are inferior to that of styrene-based thermosetting resins. In addition, 2-hydroxymethacrylate produces significant VOC emissions. Ortho and para-methyl styrene have lower volatilities than styrene; however, these chemicals still produce significant VOCs and would probably be classified as HAPs if used on a large scale.
Additives, such as paraffin waxes, have been used to suppress styrene emissions. Yet, these resins suffer from poor polymer performance and poor interfacial adhesion in fiber-matrix composites. Furthermore, studies have shown that these additives do not effectively decrease styrene emissions during the time-scale of use.
Vinyl esters are used in military and commercial applications because of their high performance, low weight, and low cost. Although it is important to reduce the styrene content in these resins, the fracture toughness of VE resins must be improved for military, automotive, and other applications. Many methods have been used to toughen these resins, but with little success.
A simple way to improve impact performance of thermosets is through matrix toughening, or decreasing the crosslink density of the network. Previous work shows that this method works for vinyl esters. Synthesizing vinyl ester monomers with a higher molecular weight decreases the crosslink density and gives the network more molecular flexibility. A more flexible network corresponds to a tougher system; however, direct losses are experienced in other mechanical and thermal properties such as modulus and glass transition temperature (Tg). In addition, increasing vinyl ester molecular weight also increases resin viscosity. This viscosity increase could prohibit the use of inexpensive liquid molding techniques for composite fabrication.
To avoid significant plasticization of the matrix, other methods for toughening can be found in the literature and have been used commercially. Second phase toughening with rubber modifiers that precipitate from solution is one such method. It has been shown that thermosetting systems, such as epoxies, can be toughened through rubber modification. For example, phase separation of a carboxyl-terminated rubber (CTBN) from a reacting mixture of diglycidyl ethers of bisphenol-A (DGEBA) and diamines (e.g. diamino diphenyl sulfone) results in a well-dispersed phase of rubber particles having typical dimensions of 1 μm and a material with improved toughness. For higher molecular weight DGEBA systems, the increase in fracture toughness is dramatic, reaching up to one order of magnitude. The rubber modifier must be miscible with the resin at room temperature and should fully precipitate from solution during cure to avoid plasticizing the epoxy phase.
Because rubber modification of epoxies has been successful, it should follow that toughening vinyl esters would experience the same type of success. Dreerman and coworkers attempted to toughen vinyl ester using an epoxy terminated (ETBN) and vinyl terminated (VTBN) butadiene-acrylonitrile rubber. They found that ETBN yielded the highest degree of toughening with approximately 70% increase in Klc. This pales in comparison to toughened epoxies, which exhibit over an order of magnitude increase in GIC from unmodified epoxies. Also the rubber modifiers were not compatible with the vinyl ester, which is a necessary condition for rubber toughening. Attempts to improve the compatibility (increased temperature, ultrasonic treatment, and surfactants) were not successful. Ullet reported similar trends when toughening vinyl ester. Two-phase mixtures were reported for butadiene-acrylonitrile based rubber modifiers. Compatibilizing agents improved the solubility of the rubber, but hindered the phase separation during cure. Also, a relatively low increase in Klc (116%) was achieved. Siebert and coworkers were able to obtain higher levels of toughness, as high as a 540% increase in KIC. Unfortunately, this toughening required relatively high rubber levels and resulted in a significant plasticization of the vinyl ester matrix. Similar problems with toughening vinyl esters were experienced by other research groups as well.
Bimodal blends of unsaturated polyester monomers have been used to make resins with good fracture properties and reduced resin viscosities. However, the viscosities of these resins were significantly higher than what is acceptable for inexpensive liquid molding of composite structures. Blends of VE monomers have also been used to make polymers. Dow Derakane resins typically contain mixtures of VE monomers with molecular weights as low as 540 g/mol and as high as 1900 g/mol. Because the molecular weights of the components are fairly similar, only modest increases in fracture toughness are observed (GIC˜250 J/m2). On the other hand, the VE multi-modal blends detailed herein have GIC values greater than 200 J/m2 at low styrene contents (˜30 wt %.). Therefore, multi-modal blends allow for the production of polymers with high fracture toughness, low viscosity, and low VOC emissions.